I'm reading this out of Barker 2nd edition Neuroscience Nursing A Spectrum of Care

"Cushing's reflex is helpful in explaining the late signs of increased ICP. This response is seen with sudden increases in pressure that result from distortion of a pressure area in the medulla beneath the floor of the fourth ventricle. When the CSF pressure rises to equal arterial pressure, the arteries become compressed and blood supply to the brain is decreased. The vasomotor center becomes ischemic and triggers the arterial pressure to rise in compensation for low perfusion. A sympathetically mediated response causes the systolic blood pressure to increase, and there is a widening pulse pressure, slowing of the pulse, and irregular respirations. Patients presenting with Cushing's reflex have recovered after rapid treatment."

I also asked a Neurosurgeon who explained it this way.....as the systolic rises but the pulse drops, it's like there is more time for the diastolic pressure to fall since the pulse is slower..not a lot of pressure built up between beats., therefore the diastolic pressure is lower...does that make sense???

The Cushing Reaction is a combination of two events in response to cerebral ischemia:

1) Central chemoreceptors--in the brain--register an increase in PCO2 (increased pH) and stimulate vasoconstriction throughout the body to increase systolic pressure. Remember that the cerebral vessels do not have a great amount of muscle and therefore are not as succeptible to vasoconstriction.

2) The rise in peripheral BP (sensed by baroceptors in the carotid sinus and aortic arch) increases the rate of afferent (to the brain) firing of the nerves running to the brainstem (CN 9 and CN 10). This causes a reflex DECREASE in the firing of efferent SYMPATHETIC (and INCREASE in firing of efferent PARASYMPATHETIC) nerves to the heart resulting in BRADYCARDIA.

In short, Hypertension is the sympathetic response, and bradycardia is the parasympathetic response.